Nitric oxide (NO) is usually overshadowed in fame by its more famous cousin laughing gas, but it’s difficult to think of many simple molecules that have such a variety of important biological functions. While NO only lasts a few seconds in the free gaseous state in the blood, it has been implicated in processes that involve everything from immune function to neurotransmission. One important role for NO is in the cardiac system, where it functions as a vasodilator and in vertebrates it slows heart rate, while in insects it has the opposite effect.

Baculum extradentatum photo by Sara da Silva

Most of the research about the physiological functions of NO has focused on vertebrates, but recent work published in the journal of Cellular Signalling by graduate student Sara da Silva and her postdoctoral fellow mentor Rosa da Silva in the lab of Angela Lange (University of Toronto Mississauga), has shown that, unlike other insects, the Vietnamese stick insect Baculum extradentatum can respond to NO like a vertebrate.

“Our initial research interests in cardiac physiology were influenced by earlier work indicating that stick insect hearts are innervated and can be modulated by endogenous chemicals [like NO],” says study director and University of Toronto Biology professor Angela Lange. “It is for this reason that we chose this understudied organism, which contains a simplified cardiovascular system that can be considered a model for work on other cardiac systems.”

The researchers first attempted to find the natural source of NO in the stick insect by removing hemolymph (blood) samples and staining for the presence of an enzyme that produces NO. Then they examined the effects of NO on heart rate by dissecting the dorsal vessel out and maintaining it in a Petri dish with physiological saline. They could measure heart rate through the placement of electrodes on either side of the dissected heart, and monitor the effects of various chemicals on the cardiac activity of the stick insect. They also could examine whether heart rate was mediated by the central nervous system by leaving the nervous system attached or not.

The effects of nitric oxide on the heart rate of B. extradentatum. Figure 3 from da Silva et al. 2012

They found that the hemocytes (blood cells) of the stick insect were producing an enzyme that was similar to the enzyme other animals use to produce NO. In addition, the more of a chemical called MAHMA-NONOate (which produces NO) they added, the slower the stick insect hearts beat. This surprising effect was completely opposite to what had been found in other insects and was more like the response of the vertebrate heart.

“Insects have evolved different strategies depending upon life history, and have co-opted different messenger systems for this success,” says study author da Silva. “We need to understand the full ecology of all species to finally appreciate the factors involved.”

Using the same setup, they also tested other components of a system of compounds that they thought might be involved in the pathway that produces NO that leads to decreased heart rate in B. extradentatum. They believe that NO is produced in the hemocytes, travels to the wall of the heart, and then leads to the production of a messenger molecule that decreases heart rate.

“This study further emphasizes the evolutionary links between the physiological processes of vertebrate and invertebrate systems,” says da Silva. “Our findings suggest that signaling molecules (such as NO) common to both types of organisms can have similar effects on cardiac activity. These novel findings demonstrate that the study of vertebrate systems can be complemented with studies in model invertebrate organisms.”

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